Somatomedin-C Enhances Induction of Luteinizing Hormone Receptors by Follicle-Stimulating Hormone in Cultured Rat Granulosa Cells*

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Abstract
Recent studies have documented the ability of somatomedin-C (Sm-C) to synergize with FSH in the induction of rat granulosa cell progesterone biosynthesis. This direct cytodifferentiation action of Sm-C proved distinct from its established mitogenic property. To determine whether Sm-C partakes in the differentiation of granulosa cell functions other than progesterone biosynthesis, the role of Sm-C in the acquisition of LH [luteinizing hormone] receptors was investigated in vitro. Granulosa cells were obtained from immature, hypophysectomized diethylstilbestrol-treated rats and cultured under serum-free conditions for up to 72 h. Specific granulosa cell LH/hCG [human chorionic gonadotropin] binding under basal conditions was relatively low and was not significantly affected by treatment with highly purified Sm-C (50 ng/ml). In contrast, treatment with FSH (oFSH; NIH FSH S14; 20 ng/ml) produced a 3.8-fold increase in LH/hCG binding to 3167 .+-. 660 (.+-. SE) cpm/106 cells. However, concurrent treatment with Sm-C (50 ng/ml) resulted in a 6.1-fold enhancement of the FSH effect. The ability of Sm-C to synergize with FSH in the induction of LH receptors was dose and time dependent, with an apparent median effective dose (ED50) of 6.2 .+-. 0.6 (.+-. SE) ng/ml, and a minimal time requirement of .ltoreq. 24 h. Examination of the binding parameters of the LH/hCG receptor revealed that the Sm-C mediated increase in LH/hCG binding was due to enhanced binding capacity, but not affinity (Kd = 4.4 .times. 10-11 M). Decreasing dilutions (1:128,000-1:8,000) of a monoclonal antibody raised against Sm-C (sm 1.2), produced progressive and virtually complete immunoneutralization of the synergistic interaction between Sm-C and FSH, suggesting specificity of action. The ability of Sm-C to enhance LH receptor induction was associated with increased hCG-stimulated progesterone biosynthesis, suggesting that the newly acquired receptors are functional in nature. Our present findings indicate that nanomolar concentrations of Sm-C are capable of enhancing FSH-stimulated LH/hCG-binding capacity, but not affinity, in a time- and dose-dependent fashion. These observations suggest that the direct cytodifferentiative effect of Sm-C is not limited to the induction of progesterone biosynthesis, but other key granulosa cell functions may also be subject to modulation by this regulatory peptide.